The invention relates to the field of solid electrochemistry.
The elementary electrochemical cell used for separating oxygen from the air or from a gas mixture containing it generally consists of a ternary system comprising solid electrolyte/electrodes/current collectors.
The solid electrolytes used for separating oxygen from a gas mixture are doped ceramic oxides which, at the operating temperature, are in the form of a crystal lattice having oxide ion vacancies. The associated crystal structures may, for example, be fluorite, perovskite or brown-millerite cubic phases called Aurivillius phases; J. C. Boivin and G. Mairesse have referred to all the crystal phases which are O2xe2x88x92 anionic conductors in a general article (Chem. Mat., 1998, pp 2870-2888; xe2x80x9cRecent Material Developments in Fast Oxide Ion Conductorsxe2x80x9d).
The electrode materials associated with the solid electrolyte are generally perovskites. These are materials having a crystal structure based on the structure of natural perovskite, CaTiO3, and exhibit good hybrid (ionic and electronic) conductivity properties by virtue of this cubic crystal structure in which the metal ions are located at the corners and at the center of an elementary cube and the oxygen ions are in the middle of the edges of this cube. The electrode materials may also be mixtures of perovskite materials and of a purely ionic conductor or else mixtures based on materials having other crystal phases, for example of the Aurivillius, brown-millerite or pyrochlore type.
The current collecting is provided either by a metal or a metal lacquer or by a metal/xe2x80x9cinert oxidexe2x80x9d ceramic (such as alumina) mixture, by a metal/carbide (such as silicon carbide) mixture or by a metal/nitride (such as silicon nitride) mixture, in which the main role of the oxide, the carbide or the nitride is to mechanically block the segregation/sintering phenomena that occur because of the high operating temperatures (700xc2x0 C. less than T less than 900xc2x0 C.), especially when silver is used as the current collector metal, or by a metal/xe2x80x9chybrid conductorxe2x80x9d oxide ceramic (such as an oxide having a perovskite structure of the family of strontium-doped lanthanum manganites) mixture or by a metal/xe2x80x9cion conductorxe2x80x9d oxide ceramic (such as yttrium-stabilized zirconia) mixture.
However, the Applicant has found that when a tubular electrochemical cell in which the solid electrolyte is zirconium oxide stabilized with 8% yttria (8% YSZ), the electrodes are made of La0.9Sr0.1MnO3xe2x88x92xcex4 (LSM) and the current collectors are a silver lacquer is operated at a temperature of between 700 and 900xc2x0 C., whether at atmospheric pressure or at an internal oxygen pressure of 20xc3x97105 Pa (120 bars) or as at an external oxygen pressure of 120xc3x97105 Pa (120 bar), accelerated ageing of this cell is observed, resulting in a 70% increase in the cell voltage in 40 h of operation; by replacing the current collectors made of silver lacquer with current collectors made of 50/50 vol % Ag/(8% YSZ) or 50/50 vol % Ag/LSM xe2x80x9ccermetsxe2x80x9d or metal/ceramic mixtures, the ageing is slowed down. However, the degradation phenomenon is not completely eliminated since a 6-15% increase in the total voltage is observed for 100 h of operation. When the cell is operated with an internal oxygen pressure of 20xc3x97105 Pa (20 bar) at 780xc2x0 C., a reduction in the coulombic efficiency and a drop in the voltage are also observed.
In the case of current collectors based on silver lacquer, it has been possible to attribute the ageing (with 1 less than P less than 20xc3x97105 Pa) and the drop in coulombic efficiency at high pressure (P greater than 20xc3x97105 Pa) and at high temperature (800xc2x0 C.) to three concomitant phenomena:
a silver sintering/segregation phenomenon for temperatures greater than 750xc2x0 C.;
a silver evaporation phenomenon accentuated by the flushing of the cell with hot air, for temperatures greater than 700xc2x0 C.; and
a silver diffusion phenomenon at pressure (20xc3x97105 Pa) through the solid electrolyte at high temperature ( greater than 780xc2x0 C.).
L. S. Wang and S. A. Barnett have described the use of LaCoO3 for covering stabilzied-zirconia-based cells which are covered with an Ag/YSZ mixture. This work has shown that, after 150 h at 750xc2x0 C., the YSZ/Agxe2x80x94YSZ(50/50)/LaCoO3 layer (1 xcexcm) system did not lose silver, unlike the system without the xe2x80x9cprotectivexe2x80x9d layer of LaCoO3 for which there was, over time, segregation and loss of silver mass by evaporation. However, perovskite LaCoO3 does not have good hybrid conductivity properties.
The Applicant has therefore sought a means of limiting, or indeed stopping, the degradation described above
This is why the subject of the invention is a ceramic membrane, which is an oxide ion conductor, characterized in that it comprises a non-zero volume, of non-zero total thickness E, of an assembly consisting of:
a) a dense layer, having opposed faces of areas S and Sxe2x80x2 and having a non-zero thickness e, of a solid electrolyte having, at the electrolysis temperature, a crystal structure which is an oxide ion conductor;
b) two porous electrodes, which are hybrid conductors and have non-zero thicknesses e1 and exe2x80x21, which are identical or different, coated on non-zero areas s1 and sxe2x80x21, which are identical or different, of the two opposed faces of areas S and Sxe2x80x2 of the said solid electrolyte;
c) two porous current collectors, of non-zero thicknesses e2 and exe2x80x22, which are identical or different, coated on non-zero areas s2 and sxe2x80x22, which are identical or different, of the said two porous electrodes; and
d) at least one porous covering layer, of non-zero thickness e3, coated on a non-zero area s3, of at least one of the said collectors, made of a material, or of a mixture of materials, which is chemically compatible with the materials, or the mixture of materials, of the said electrodes, the said collectors and the said solid electrolyte, and the sintering temperature of which is very close to the sintering temperatures of the materials, or of the mixtures of materials, of which the said electrodes, the said collectors and the said solid electrolyte are composed, and characterized in that the thickness E of the said membrane is equal to the sum of the thicknesses of each of the elements mentioned.
The expression xe2x80x9ccrystal structure which is an oxide ion conductorxe2x80x9d should be understood within the context of the present invention to mean any crystal structure which, at the operating temperature, is in the form of a crystal lattice having oxide ion vacancies. The associated crystal structures may, for example, be fluorite, perovskite, brown-millerite cubic phases called Aurivillius phases or else those mentioned in: J. C. Boivin and G. Mairesse, Chem. Mat., 1998, pp 2870-2888; xe2x80x9cRecent Material Developments in Fast Oxide Ion Conductorsxe2x80x9d.
The expression xe2x80x9cmaterial or mixture of materials, which is chemically compatible with that of the current collector or collectorsxe2x80x9d should be understood in the present description to mean any material or mixture of materials which, at a sintering temperature of between approximately 600xc2x0 C. and 1000xc2x0 C., does not undergo any chemical reaction with that material or those materials of the layer which it covers, namely in the present case, the material or mixture of materials of which the current collector(s) is(are) composed. Such a chemical reaction would possibly be revealed by the appearance of one or more chemical compounds absent in the initial materials or mixtures of materials.
The expression xe2x80x9cporous layersxe2x80x9d means, in the present description, that the layers of materials in question must be capable of allowing dioxygen to diffuse. More generally, their porosity index is between 10% and 70%, more precisely between 30 and 60%.
The expression xe2x80x9chybrid conductorsxe2x80x9d in the present description means that the layers of materials in question are both ion and electron conductors.
The expression xe2x80x9cvery similar sintering temperaturesxe2x80x9d means that the difference between the sintering temperatures of the porous covering layer and of the current collector is less than or equal to approximately 200xc2x0 C. When this difference becomes too great, a delamination phenomenon, indicating poor adhesion of the sintered layers, is observed.
The subject of the invention is especially a ceramic membrane, as defined above, comprising two covering layers of thicknesses e3 and exe2x80x23, which are identical or different, coated on non-zero areas s3 and sxe2x80x23, which are identical or different, of each of the said current collectors, and characterized in that the thickness E of the said volume of the said membrane is equal to the sum of the thicknesses e+e1+exe2x80x21+e2+exe2x80x22+e3+exe2x80x23 and more particularly a ceramic membrane characterized in that e1=exe2x80x21, e2=exe2x80x22 and, where appropriate, e3=exe2x80x23.
In the ceramic membrane as defined above, e generally ranges between approximately 0.25 mm and approximately 2 mm and more particularly between approximately 0.5 mm and approximately 1 mm, e1 and exe2x80x21, generally range between approximately 1 xcexcm and approximately 50 xcexcm and more particularly between approximately 10 xcexcm and approximately 30 xcexcm, e2 and exe2x80x22 generally range between approximately 1 xcexcm and approximately 100 xcexcm and more particularly between approximately 20 xcexcm and approximately 60 xcexcm and e3 and, where appropriate, exe2x80x23, generally range between approximately 1 xcexcm and approximately 200 xcexcm and more particularly between approximately 20 xcexcm and approximately 100 xcexcm.
According to a first particular embodiment, the subject of the invention is a ceramic membrane, as defined above, consisting of a sheet having plane faces of areas S and of thickness E and especially a sheet of length L ranging between approximately 1 cm and approximately 1 m and more particularly between 5 cm and approximately 50 cm and of width 1 ranging between approximately 1 cm and approximately 1 m and more particularly between 5 cm and approximately 50 cm.
According to a second particular embodiment, the subject of the invention is a ceramic membrane, consisting of a cylinder of external diameter D and internal diameter d, characterized in that the support layer for the said cylinder is the cylindrical dense layer, of thickness e, of solid electrolyte and in that half the difference Dxe2x88x92d is equal to the sum of the thicknesses e, e1, exe2x80x21, e2, exe2x80x22 and e3, and possibly exe2x80x23 and, more particularly, a cylindrical ceramic membrane of length L ranging between approximately 1 cm and approximately 1 m and more particularly between 10 cm and 50 cm.
The solid electrolytes used in the ceramic membrane forming the subject of the present invention are generally doped ceramic oxides which, at the operating temperature, are in the form of a crystal lattice having oxide ion vacancies. The compounds most conventionally used have a fluorite structure. These oxides are represented more particularly by the formula (I):
(Mxcex1Oxcex2)1xe2x88x92x(Rxcex3Oxcex4)xxe2x80x83xe2x80x83(I)
in which M represents at least one trivalent or tetravalent atom mainly chosen from bismuth (Bi), cerium (Ce), zirconium (Zr), thorium (Th), gallium (Ga) and hafnium (Hf), xcex1 and xcex2 are such that the Mxcex1Oxcex2 structure is electrically neutral, R represents at least one divalent or trivalent atom chosen mainly from magnesium (Mg), calcium (Ca), barium (Ba) and strontium (Sr), or gadolinium (Gd), scandium (Sc), ytterbium (Yb), yttrium (Y), samarium (Sm), erbium (Er), indium (In), niobium (Nb) and lanthanum (La), xcex3 and xcex4 are such that the Rxcex3Oxcex4 structure is electrically neutral and x generally ranges between 0.05 and 0.30 and more particularly between 0.075 and 0.15.
A solid electrolyte may consist, for example, of a single MO2 oxide combined with one or more Rxcex3Oxcex4 oxides or else of a mixture of oxides MO2 which is combined with one or more Rxcex3Oxcex4 oxides. As ceramic oxides of formula Mxcex1Oxcex2, there are principally zirconium oxide (ZrO2), cerium oxide (CeO2), hafnium oxide (HfO2), thorium oxide (ThO2), gallium oxide (Ga2O3) and bismuth oxide (Bi2O3). These oxides are doped with one or more oxides chosen generally from magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO), strontium oxide (SrO), gadolinium oxide (Gd2O3), erbium oxide (Er2O3), indium oxide (In2O3), niobium oxide (Nb2O3), scandium oxide (Sc2O3), ytterbium oxide (Yb2O3), yttrium oxide (Y2O3), samarium oxide (Sm2O3) and lanthanum oxide (La2O3). As main example of solid electrolytes, there are zirconias (zirconium oxides), gallates (materials based on gallium oxide), BIMEVOX-type materials and stabilized zirconium oxides such as, for example, stabilized zirconia of formula (Ia):
(ZrO2)1xe2x88x92x(Y2O3)xxe2x80x83xe2x80x83(Ia),
in which x ranges between 0.05 and 0.15, called hereafter YSZ(x in mol %). These compounds operate at temperatures ranging between 700 and 800xc2x0 C.
The electrodes associated with the solid electrolyte, having identical or different compositions, are especially made of a material or of a mixture of materials having a perovskite (ABO3) or similar structure (pyrochlore (A2B2O7), brown-millerite (A2B2O5) and BIMEVOX (Aurivillius phases)). Perovskite materialsxe2x80x94the main electrode materials, are represented by the formula (II):
M1M2O3xe2x80x83xe2x80x83(II)
in which M1 represents one or more atoms chosen from families of alkaline earths, lanthanides and actinides and more particularly chosen from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y or Mg, Ca, Sr and Ba, M2 represents one or more atoms chosen from the transition metals, more particularly chosen from Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. Within the context of the present invention, the electrodes, of identical or different compositions, are more particularly chosen from lanthanum nickel oxide (LaNiO3), calcium lanthanum manganites (CauLavMnOw), lanthanum strontium manganites (LauSrvMnOw), lanthanum strontium cobaltites (LauSrvCoOw), calcium lanthanum cobaltites (CauLavCoOw), gadolinium strontium cobaltites (GduSrvCoOw), lanthanum strontium chromites (LauSrvCrOw) lanthanum strontium ferrites (LauSrvFeOw) and lanthanum strontium ferrocobaltites (LauSrvCodFecOw), in which u+v and c+d are equal to 1 and w is such that the structure in question is electrically neutral.
The current collectors, coated on the said two porous electrodes, of identical or different compositions, essentially consist either of a metal or of a metal lacquer, such as a gold lacquer or a silver lacquer for example, or of a metal/xe2x80x9cinert oxidexe2x80x9d ceramic (such as alumina) mixture or of a metal/xe2x80x9chybrid conductorxe2x80x9d oxide ceramic (such as a perovskite material) mixture or of a metal/xe2x80x9cion conductorxe2x80x9d oxide ceramic (such as (8 mol %) yttrium-stabilized zirconia) mixture or of a metal/xe2x80x9celectron conductorxe2x80x9d oxide ceramic (such as nickel oxide) mixture or of a metal/carbide (such as silicon carbide) mixture or of a metal/nitride (such as silicon nitride) mixture. The metal used in the current collectors is mainly chosen from transition metals, more particularly from silver, copper and nickel or from noble metals, more particularly from gold, platinum and palladium. They may also be current collector wires based on oxidizable materials but covered with a thin layer of gold, silver or platinum. The current collectors are more particularly made of a mixture of a metal chosen from silver and gold, with one or more compounds of formula (I) as defined above or of a mixture of a metal chosen from silver and gold, with one or more compounds of formula (II) as defined above. The two current collectors most particularly have an identical composition and are made of a mixture of silver and a xe2x80x9cion conductorxe2x80x9d ceramic such as yttria-doped zirconia such as YSZ(8%) for example. Each of the current collectors is connected to the external part of the circuit by an electronically conducting wire, often made of a metal identical to that of which the said collector is composed.
According to a variant of the present invention, the cylindrical ceramic membrane as defined above is filled with beads of mullite or of zirconia, so as to improve the fastening of the said wire to the said current collector. The nature of the beads may also be of the metallic or metal carbide type, or with beads of mullite or zirconia, covered with a current collector layer having the same nature as or a different nature from the current collector layer of the tubular electrochemical cell.
The covering layer, coated on at least one of the said collectors, may be a hybrid or an electron conductor or may be insulating. When it is insulating, it may, for example, be an enamel. When it is a hybrid conductor, it may, for example, be a perovskite material or a mixture of perovskite materials or a mixture of perovskite materials or materials of similar families (pyrochlores or brown-millerites) and of purely ionic conductors, and more particularly a compound or a mixture of compounds of formula (II) as defined above. The subject of the invention is particularly a ceramic membrane as defined above, in which the covering layer, coated on at least one of the said collectors, is a compound of formula (IIa):
La0.8Sr0.2Co0.8Fe0.2Owxe2x80x83xe2x80x83(IIa)
in which w is such that the structure of formula (IIa) is electrically neutral. When there is a covering layer coated on each of the current collectors, these are of identical or different compositions.
The ceramic membrane forming the subject of the present invention is prepared by successive sequences consisting of the deposition of a given commercially available material, followed by the sintering of the resulting combination, using the solid electrolyte as material for supporting the said membrane. These sequences of operations are well known to those skilled in the art. In general, the successive deposition operations are carried out by painting, by spraying, by dip coating or by screen printing, whether on the internal face or on the external face of the device. After each layer has been deposited, the sintering is carried out in air, at the sintering temperature of the said material ranging between 600xc2x0 C. and 1000xc2x0 C., depending on the materials, for several hours, generally from 0.5 to 10 hours. Likewise, the solid electrolyte, namely a ceramic membrane of tubular, planar or elliptical geometrical shape, is prepared from commercial products and formed using methods known to those skilled in the art.
According to a last aspect of the present invention, the subject of the latter is the use of a ceramic membrane as defined above, for separating oxygen from the air or from a gas mixture containing it.